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A “roadmap” to a fuel for the future: AECL continues research into thoria fuels

Over the past few years, interest in thorium (Th) as fuel alternative to uranium has grown. Thoria fuels are not new to AECL; in fact, as Canada’s premier nuclear science and technology laboratory, AECL has been researching and developing them for decades.

It is abundant. Known resources of thorium far exceed those of uranium.

It is secure. Unlike uranium, thorium does not contain a fissile component; however, thorium is “fertile” – it produces fissile U-233 under neutron irradiation, making it suitable as a fuel. It is safe. Ceramic thoria fuels (i.e., thorium-dioxide, ThO2) have several advantages over conventional UO2 fuels, including higher thermal conductivity resulting in lower fuel operating temperatures. And, unlike UO2, ThO2 is chemically stable; it does not oxidize when exposed to water/steam/air at high temperature.

These differences translate into important fuel performance improvements under normal operating conditions and in postulated accident scenarios. For these reasons and many others, thoria fuels are not only of interest to CANDU® operators, but also to other reactor types such as the Super Critical Water Reactor.

Although AECL has been conducting research on thoria fuels for many years, there are still gaps in our fundamental understanding, particularly around thoria-based fuels that contain added fissile components such as uranium or plutonium. Gap closure requires long lead times for activities such as irradiation testing and post-irradiation examination; as a result, it is important to identify and address gaps in a timely manner, as we approach greater implementation of thoria-based fuels in commercial nuclear power reactors.

Because thoria has no initial fissile component, it must to be accompanied in a reactor by a fissile component such as U-235 or Pu-239 in order to start “breeding” fissile U-233. This can be done in a variety of ways. One is to have some of the fuel elements in a bundle contain “natural thoria” (pure ThO2) and other elements contain UO2 that is slightly enriched in U-235 (SEU). Another way is to blend U-235 or plutonium (primarily Pu-239) with the thoria resulting in (Th, U)O2 or (Th, Pu)O2 fuels. Adding UO2 or PuO2 to ThO2 changes the materials properties of the thoria-based fuel and, potentially, the behaviour of the fuel – this is one aspect that AECL is exploring. This is of particular interest when the quantity of the added fissile component is relatively large (as in the Canadian Super Critical Water Reactor (SCWR) fuel design.

To identify and address gaps in thoria fuel science and technology, AECL’s Fuel Development Branch initiated the multi-year “Thoria Roadmap Project”. The first phase, which began in 2011, will be to determine what fundamental understanding will be required to support future thoria fuel cycles and designs for various reactor types. The second phase (initiated in 2013) will be to determine the status of current understanding both internal and external to AECL to determine what “gaps” exist between our envisioned future needs and the existing knowledge base. The third phase (2014/2015) will be to create a roadmap - a plan to address gaps in thoria fuel science and technology - while the fourth phase will be to execute the plan.

The Thoria Roadmap Project will involve collaborations with universities and international organizations, working alongside staff from various disciplines within AECL. Projects such as this are part of AECL’s mandate as national nuclear science and technology organization that provides benefits to Canada and the world. This initiative will assist Canada and the world in ensuring a safe, reliable and secure nuclear energy source for future generations.